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Abstract

The front-coating (FC) of a solar cell controls its efficiency, determining admission of light into the absorbing material and potentially trapping light to enhance thin absorbers. Single-layer FC designs are well known, especially for thick absorbers where their only purpose is to reduce reflections. Multilayer FCs could improve performance, but require global optimization to design. For narrow bandwidths, one can always achieve nearly 100% absorption. For the entire solar bandwidth, however, a second FC layer improves performance by 6.1% for 256 μm wafer-based cells, or by 3.6% for 2 μm thin-film cells, while additional layers yield rapidly diminishing returns.

Figures (9)

(Color online) Schematic illustration of solar cell designs studied in this paper: (a) a photovoltaically active silicon region (green), backed by a perfectly reflecting metal (gray), (b) diagram (a) with one or more front coating depicted in shades of blue, and (c) diagram (b) with one back dielectric coating layer.

(Color online) A contour plot of the FOM of a cell with two front-coating layers versus the layer thickness (the indices of each layer are fixed at 1.27 and 4.35). The FOM ranges from 0.30 (black) to 0.38 (white). Clearly there are many local optima, necessitating a global optimization approach.

(Color online) The generated current efficiency as a function of wavelength; the parameters for each structure class (e.g., two front layers) are optimized separately at each wavelength i.e., this is the reflection spectrum for many different structures. Note that structures with one or more front layers display full absorption up to a particular wavelength λt that increases with the number of front layers.

(Color online) The generated current efficiency versus the bandwidth of incoming radiation; for bandwidths up to 265 nm the central wavelength is 902.8 nm, for band-widths above 265 nm, the maximum wavelength is fixed, while the minimum wavelength is decreased (which corresponds to a blue shift of the central wavelength).

(Color online) Absorption spectrum over the full absorbing bandwidth for a thin-film crystalline silicon solar cell (t = 2μm). (a) Optimized over the full bandwidth and (b) optimized only at λ = 902.8 nm.

(Color online) Relative difference in figure of merit, calculated as the relative difference of the FOM for the optimized structure with the given reflection phase and the FOM of a structure with the given phase and with the optimized front coating of the reference phase (θ= 0), versus the Fresnel reflection amplitude phase of the silicon-metal boundary.

(Color online) Absorption spectrum of the optimized single front coating reference structure for two different back-reflector phases (π/4 blue, and π red). The back-reflector simply shifts the peaks, but has a only negligible effect on the integrated absorption or the FOM.